The invention relates to a low-pressure mercury discharge lamp provided with a tubular discharge vessel having a tube axis and enclosing a discharge space containing a filling of mercury and at least one inert gas in a gastight manner. Current supply conductors extend from outside the discharge vessel to electrodes arranged inside the discharge vessel, which electrodes each have a first and a second fastening. At least one of said electrodes is surrounded by a screen which has a smallest width W, transverse to the direction from the first to the second fastening, and in a plane transverse to the tube axis, which width is smaller than a distance D between said fastenings.
Such a low-pressure mercury discharge lamp, also referred to as lamp hereinafter, is known from U.S. Pat. No. 4,891,551. The discharge vessel has an electrode on either side in a commercially available lamp of this type. Each of the electrodes is surrounded by a screen having a smallest width W of 7 mm and a length L of 5 mm. The electrodes are fastened to their current supply conductors, the distance D between the first and the second fastening being 10 mm. Such lamps may be integrated with a supply unit so as to form a lighting unit, or alternatively it may be possible for them to be detachably coupled to a supply unit. A supply unit which ignites the lamp in the cold state is attractive on account of its simplicity. Moreover, the lamp emits light immediately in the case of cold ignition. It was found, however, that the known lamp consumes much mercury in the case of cold ignition. This is particularly disadvantageous in applications where the lighting unit is switched on frequently. Mercury consumption is here understood to be the phenomenon that mercury from the discharge space is bound during lamp life, so that it is no longer available for the discharge.